An example of a conveying apparatus, which conveys materials, parts, semi-finished products, finished products, and other such products in a production facility or the like, is one in which a ball bearing (it is also referred to as “ball transfer”), which is structured so as to provide one ball that rotates freely in all directions and is in direct contact with the conveyed products, is arranged in plurality, and thereby products are conveyed by being smoothly slid. Although this type of ball bearing has various uses, the ball bearing generally has the structure of the ball bearing 1 shown in FIG. 3. In this structure, the ball supporting member 2 has a hemispherical concave surface 2a on which a plurality of small balls 3 is freely arranged. One large ball 4 is seated thereon, and the large ball 4 is retained so as to be rotatable in all directions. All of these components are accommodated inside a casing 5, which has an aperture 5a through which the upper portion of the large ball 4 projects.
In this type of ball bearing 1, it is assumed that in order for the large ball 4 to be able to rotate smoothly without resistance, it is necessary that the plurality of small balls 3 supporting the large ball 4 all have an identical diameter to a high precision. Therefore, in the conventional ball bearing 1, as illustrated in the figure, all of the small balls 3 have an identical diameter.
There are various environments in which the ball bearing 1 may be used. Because these environments are not limited to those free of contaminants, contaminants may penetrate inside the ball bearing. When this occurs, the contaminants adhere to the small balls 3, the smoothness of the rotation of the small balls 3 deteriorates, the smoothness of the rotation of the large ball 4 deteriorates, and thereby the performance as a ball bearing degrades. In addition, the smoothness of the rotation of the large ball 4 deteriorates when contaminants adhere directly to the large ball 4 or when contaminants become interposed between the large ball 4 and the small balls 3.